1. Technical Field
The present invention relates to a light emitting device including a light emitting element, and more particularly, to a light emitting device having high light extraction efficiency of light emitted from the light emitting element.
2. Background Art
Among ways to improve the light extraction efficiency of a light emitting device, enhancement of the reflectance in a package where a light emitting element to be mounted has been actively conducted. Provision of a metal film on the light incident surface receiving light from the light emitting element is one of the known methods to enhance the reflectivity, as disclosed in JP2006-324438A for example. In the light emitting device disclosed in JP2006-324439A, silver plating is applied on the mounting surface, which is a heat sink, so as to use the mounting surface as a light reflecting member, thus improves the light extraction efficiency of the light from the light emitting element.
In a case where a light emitting element is mounted on a light emitting device, typically, the light emitting element is die-bonded to a package having a lead electrode made of a metal plate such as Cu, and the light emitting element and the lead electrode are wire-bonded with a conductive wire made of Au and the like. At this time, if a blue light emitting element is used as the light emitting element, the extraction efficiency of the light emitting device will be decreased, due to attenuation of blue light by absorption by Cu or Au. Particularly, the conductive wire is connected directly to the light emitting element and also placed above, across the light emitting surface side of the light emitting element. Therefore, absorption of light by the conductive wire is not negligible. If the conductive wire is made of a metal (e.g. Ag) having a high reflectivity for blue light, absorption of light by the conductive wire can be greatly reduced. However, bonding between the light emitting element and the lead electrode is low and sufficient bonding strength cannot be obtained.
A composite bonding wire formed with a core material and a covering material covering the core material is known as a conductive wire, which can be used for wire bonding in a semiconductor device, as disclosed in JP H03-32033A, JP H07-66235A, and JP H10-130882A for example. In these composite wires, a high-strength material is used for the core material to enhance mechanical strength of the composite wire, and an electrically conductive material which can easily bond with the covering material is used to ensure bonding strength and electrical conductivity.
Accordingly, it is considered to suppress absorption of light by the conductive wire while maintaining sufficient bond strength by using a composite wire in the light emitting device. For example, with a composite wire formed with a core material made of Au covered by a covering material made of Ag, bond strength can be secured by Au and blue light can be reflected by Ag.
In a case where a light emitting device is fabricated using a composite wire, the expected degree of improvement in the light extracting efficiency of the light emitting device cannot be achieved.
In a case where a first bond 150 of ball bonding is formed by using a composite wire as shown in FIG. 14, even when a composite wire 140 formed of the core material 142 made of Au covered with the covering material 144 made of Ag is used, the covering member 144 does not exist on the surface of the first bond 150. This is because at the time of forming the first bond 150, the core material 142 and the covering member 144 are melted and mixed together when an end portion of the composite wire 140 is melted into a ball shape. As shown in the figure, when the first bond 150 is connected to the electrode 112 of the light emitting element 110, Au exposed on the surface of the first bond 150 absorbs light near the light emitting element 110, so that the light extraction efficiency cannot be improved efficiently.
A second bond 160 of ball bonding shown in FIG. 15 is connected by thermally compressing the composite wire 140 to a lead electrode 130, but at this time, a part of the covering material 144 subjected to the thermal compress may be detached to expose the core material 142.